But you'll want to hit soul reaper before 35% so the target drops to execute range just a split second before the 5s debuff runs out, and you have to time this based off of the target hp and the damage being dealt to it.

I just used tellmewhen to notify me when the boss was at 36% health, Soul Reaper was available, and I had a blood rune available. It was pretty easy to set up and works well. I guess you could just stare at a health bar but as blood I would rather be paying attention to what is going on around me to see if there are any adds or loose mobs that I need to pick up. I don't understand why some people see it as a flaw to have an extra notification, can't hurt anything other than silly pride imo...

I use weakauras to make the soul reaper button glow when my target hits 36%. Very easy to setup, just define an action for the glow with trigger at <=36%. You can pop up a huge image too if you like, I find the glow is sufficient.

I don't have the "rune is available" conditional defined. I want to know that I should be hitting the button as soon as I have the resources to do so.

You definitely want to hit SR well before 36% in most cases, particularly soloing. This is just a reminder so I don't forget to use it.

As Frost, I would probably not use the rune available condition either. I started using it as blood because I did not want to use my death runes on Soul Reaper. I would much rather hang on to them for a DS if I get a nasty spike or something.